在不均匀牵张大鼠膈肌标本上新霉素对接头作用的进一步研究

在不均匀牵张的大鼠离体膈肌标本上,同时记录同一终板的小终板电位(MEPP),终板电位(EPP)和微电泳外源性乙酰胆碱(ACh)诱发的乙酰胆碱电位(AChP)。以AChP幅度为突触后膜兴奋性大小指标,对MEPP和EPP进行校正;以量子含量M和二项统计参数即刻可释放量子贮存n及平均量子释放机率P为突触前递质释放指标,定量观察了新霉素(neomycin)对突触前和突触后的作用。结果表明新霉素(30μmol/L,100μmol/L)作用20min,对AChP,MEPP无影响,M_l,M_x,n和P明显下降,串EPPs(50Hz)出现脱落现象,表明此浓度下的新霉素对接头的作用主要是抑制突触前诱发的递质释放,对自发释放的量子大小并无影响。其机理可能是新霉素拮抗Ca~(2+)内流,减小n和P,从而降低诱发的递质释放。

Further study on effects of neomycin on nonuniform stretched muscle preparation of rat diaphragm

It has been shown that neomycin inhibits neuromuscular transmission both pre- and post-synaptically. Many papers reported that neomycin's presynaptic effect is predominant, because neomycin affects quantal release first and then reduces the amplitude of MEPP. In these experiments, quantal size is considered unchangeable and the amplitude of MEPP, which is contributed by both pre synaptical quantal size and the sensitivity of post-synaptic membrane, is regarded as an indirect indication of the sensitivity of post-synaptic membrane. But there have no direct experiments showing whether neomycin has any effect on quantal size (q). Still the mechanism of neomycin's pre-synaptic effect is not very clear. So it is necessary to give direct evidence for the effect of neomycin on quantal size and the relative importance of neomycin's pre- and post-synaptic effects is then to be reconsidered. In this study, on the isolated nonuniform stretched muscle preparation (INSMP) of the rat diaphragm, miniature endplate potential (MEPP), endplate potential (EPP) and acetylcholine potential (AChP) induced by injecting exogenous ACh microiontophoretically could be simultaneously recorded in the same endplate. Direct method was used to estimate the quantal release in the experiment. This design of test can be used to analyse both pre- and post-synaptic compounds of the amplitudes of MEPPand EPP. Using the amplitude of AChP as direct indication of the sensitivity of the post-synaptic membrane and quantal size (q), quantal content of the first EPP(M1), quantal content of mean EPP (Mx), and binomial statistic parameters: immediate available of transmitter store (n) and mean probability of quantal release (P), as indications of pre- synaptic quantal release, we quantitatively analysed the pre- and post-synaptic effects of neomycin on the rat neuromuscular junction. 20 min after drug administration, 30 μmol / L and 100 μmol / L neomycin had no effect on amplitudes of both AChP and MEPP, but 10 min after administration of 100μmol/L neomycin, M1, Mx, n and P decreased significantly. And a random fluctuation in a train of EPPs (50 Hz) was revealed. Failure in a train of EPPs was then revealed after 20 min. It was suggested that neomycin, in this range of concentrations primarily reduces the evoked quantal release presynaptically but has no effect on spontaneous quantal size. The mechanism may be that neomycin competes against the influx of calcium into presynaptic membrane of the nerve terminal during indirect stimulation and then reduces the evoked transmitter release.